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1. Long-lived partial melt beneath Cascade Range volcanoes

   https://doi.org/10.1038/s41561-024-01630-y  

   Pang, Guanning; Abers, Geoffrey A; Moran, Seth C; Thelen, Weston A (February 2025, Nature Geoscience)

   Quantitative estimates of magma storage are fundamental to evaluating volcanic dynamics and hazards. Yet our understanding of subvolcanic magmatic plumbing systems and their variability remains limited. There is ongoing debate regarding the ephemerality of shallow magma storage and its volume relative to eruptive output, and so whether an upper-crustal magma body could be a sign of imminent eruption. Here we present seismic imaging of subvolcanic magmatic systems along the Cascade Range arc from systematically modelling the three-dimensional scattered wavefield of teleseismic body waves. This reveals compelling evidence of low-seismic-velocity bodies indicative of partial melt between 5 and 15 km depth beneath most Cascade Range volcanoes. The magma reservoirs beneath these volcanoes vary in depth, size and complexity, but upper-crustal magma bodies are widespread, irrespective of the eruptive flux or time since the last eruption of the associated volcano. This indicates that large volumes of melts can persist at shallow depth throughout eruption cycles beneath large volcanoes. 

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2. Resolving Continental Magma Reservoirs With 3D Surface Wave Tomography

   https://doi.org/10.1029/2022GC010446  

   Maguire, Ross; Schmandt, Brandon; Chen, Min; Jiang, Chengxin; Li, Jiaqi; Wilgus, Justin (August 2022, Geochemistry, Geophysics, Geosystems)

   Abstract Surface wave tomography is widely used to improve our understanding of continental magma reservoirs that may be capable of fueling explosive volcanic eruptions. However, traditional surface wave tomography based on inversions for phase velocity maps and locally 1D shear velocity may have difficulty resolving strong 3D low‐velocity anomalies associated with crustal magma reservoirs. Here, we perform synthetic tomography experiments based on 3D seismic waveform simulations to understand how the limitations of surface wave tomography could affect interpretations of tomography in volcanic settings. We focus our modeling on the Yellowstone volcanic system, one of the largest and most thoroughly studied continental magmatic systems, and explore scenarios in which the maximum shear velocity anomaly associated with the crustal magma reservoir ranges between −10% and −66%. We find that even with the well‐instrumented setting near Yellowstone, the recovered shear velocity anomalies in the mid‐to‐upper crust are severely diminished due to the small spatial scale of the reservoir with respect to the seismic wavelengths that sample it. In particular, recoveredV_Sanomalies could be reduced by a factor of two or more, implying that the inferred melt fraction of large‐scale continental magma reservoirs may be considerably underestimated. 

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3. Magma accumulation beneath Santorini volcano, Greece, from P-wave tomography

   https://doi.org/10.1130/G47127.1  

   McVey, B.G.; Hooft, E.E.E.; Heath, B.A.; Toomey, D.R.; Paulatto, M.; Morgan, J.V.; Nomikou, P.; Papazachos, C.B. (December 2019, Geology)

   null (Ed.)

   Abstract Despite multidisciplinary evidence for crustal magma accumulation below Santorini volcano, Greece, the structure and melt content of the shallow magmatic system remain poorly constrained. We use three-dimensional (3-D) velocity models from tomographic inversions of active-source seismic P-wave travel times to identify a pronounced low-velocity anomaly (–21%) from 2.8 km to 5 km depth localized below the northern caldera basin. This anomaly is consistent with depth estimates of pre-eruptive storage and a recent inflation episode, supporting the interpretation of a shallow magma body that causes seismic attenuation and ray bending. A suite of synthetic tests shows that the geometry is well recovered while a range of melt contents (4%–13% to fully molten) are allowable. A thin mush region (2%–7% to 3%–10% melt) extends from the main magma body toward the northeast, observed as low velocities confined by tectono-magmatic lineaments. This anomaly terminates northwest of Kolumbo; little to no melt underlies the seamount from 3 to 5 km depth. These structural constraints suggest that crustal extension and edifice loads control the geometry of magma accumulation and emphasize that the shallow crust remains conducive to melt storage shortly after a caldera-forming eruption. 

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4. Stacked sills forming a deep melt-mush feeder conduit beneath Axial Seamount

   https://doi.org/10.1130/G47223.1  

   Carbotte, Suzanne M.; Arnulf, Adrien; Spiegelman, Marc; Lee, Michelle; Harding, Alistair; Kent, Graham; Canales, Juan Pablo; Nedimović, Mladen (April 2020, Geology)

   Abstract Magmatic systems are composed of melt accumulations and crystal mush that evolve with melt transport, contributing to igneous processes, volcano dynamics, and eruption triggering. Geophysical studies of active volcanoes have revealed details of shallow-level melt reservoirs, but little is known about fine-scale melt distribution at deeper levels dominated by crystal mush. Here, we present new seismic reflection images from Axial Seamount, northeastern Pacific Ocean, revealing a 3–5-km-wide conduit of vertically stacked melt lenses, with near-regular spacing of 300–450 m extending into the inferred mush zone of the mid-to-lower crust. This column of lenses underlies the shallowest melt-rich portion of the upper-crustal magma reservoir, where three dike intrusion and eruption events initiated. The pipe-like zone is similar in geometry and depth extent to the volcano inflation source modeled from geodetic records, and we infer that melt ascent by porous flow focused within the melt lens conduit led to the inflation-triggered eruptions. The multiple near-horizontal lenses are interpreted as melt-rich layers formed via mush compaction, an interpretation supported by one-dimensional numerical models of porous flow in a viscoelastic matrix. 

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5. Melt Migration in Crystal Mushes by Viscous Fingering: Insights From High‐Temperature, High‐Pressure Experiments

   https://doi.org/10.1029/2022JB024447  

   Ryan, Amy G.; Hansen, Lars N.; Zimmerman, Mark E.; Pistone, Mattia (August 2022, Journal of Geophysical Research: Solid Earth)

   Abstract We conducted experiments to study melt migration in crystal‐rich mushes, with application to magma ascent within transcrustal magma reservoirs. Mushes with crystal volume fractions of 0.59–0.83 were prepared by hot‐pressing crushed borosilicate glass mixed with different proportions of quartz sand particles. Each experimental sample comprises stacked disks of mush and soda‐lime glass, a proxy for crystal‐free magma. Samples were subjected to confining pressures of 100–300 MPa and a temperature of 900°C (above the glass transition temperatures of the borosilicate and soda‐lime glasses) for up to 6 h. The bottom and circumference of the mush and soda lime disks experience the confining pressure, but the top of the mush disks is at room pressure, resulting in a pore‐pressure gradient across the mush layer. Following cooling and decompression, we determined the area fraction and morphology of soda‐lime melt that migrated into the mush layer during experiments. Melt fraction is more strongly correlated to crystal fraction than pore‐pressure gradient, increasing with crystal fraction before sharply decreasing as crystal fractions exceed 0.8. This change at 0.8 coincides with the transition from crystals in the mush moving during soda‐lime migration to crystals forming a continuous rigid network. In our experiments, melt migration occurred by viscous fingering, but near the mobile‐to‐rigid transition, melt migration is enhanced by additional capillary action. Our results indicate that magma migration may peak when rigid mushes “unlock” to become mobile. This transition may mark an increase in magma migration, a potential precursor to volcanic unrest and eruption. 

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